In the field of digital-to-analog conversion technology for audio, the conversion rate is typically low, however, the accuracy requirement is getting more and more stringent. The audio application specifications demand multi-channel playback much more than before. Therefore, it is a task to achieve the purpose of accuracy and multi-channel output performance while maintain the product competitiveness (low cost).
The most important considerations under the accuracy specifications include the dynamic range (DR) and the signal to noise plus distortion ratio (SNDR). The dynamic range (DR) is defined as a measurement of the noise generated from the digital-to-analog converters, and the signal-to-noise-plus-distortion ratio (SNDR) is indicated as a measurement of converters linearity. There are several of digital-to-analog converters. According to the classification of conversion rate, there are two types of the devices, one is Nyquist-rate converters and another one is over-sampled converters.
In the Nyquist-rate converters, one of the straightforward implantation is so called resistor string voltage division DACs (digital-to-analog converters), another way is resistor string DACs. The primary drawback of the resistor string DACs is that the string resistor matching is limited by the VLSI technology. The resistance mismatch of the string resistor is caused by the process deviation and it will directly influence the voltage division accuracy of the resistor string. The incorrect voltage division will cause poor performance on the SNDR in the resistor string DACs. Therefore, the resistor string DACs is seldom employed in the conventional high resolution DAC.
However, resistor string DAC has some advantages. One of the advantages is that the device has the capability to provide high dynamic range (DR). The primary noise source of the resistor string DACs arises mainly from the resistor string thermal noise, switch and output buffer thermal noise and 1/f noise. Hence, its noise floor is extremely low and the characteristic of dynamic range (DR) is excellent. The further benefit for the resistor string DACs is that the device can be operated at high speeds, thereby achieving the facility of high sample rate conversion application.
The major over-sampled digital-to-analog converter is the Sigma-delta digital-to-analog converter, and the device shares a big marketing.
FIG. 1 shows a block diagram for typical single channel sigma-delta DAC (digital-to-analog converter) in accordance with the prior art. The binary digit of the input could be N bits (N may be 16, 18, 20, 24) Nyquist-rate PCM digital audio source. The sample rate FN—in is interpolated by an interpolator 100 to increase the sampling rate to R times, namely, RFN—in. The N bits RFN—in data is then reduced the wordlength to M-bit per sample by a sigma-delta modulator 110, M is smaller than N, the quantization noise generated during the procedure will be shifted to the high frequency outside the baseband via the loop.
The M-bits output of the sigma-delta modulator 110 is subsequently transformed to a staircase analog signal by M-bits digital-to-analog converters. Finally, the staircase analog signal is filtered out-of-band noise by a switched-capacitor low-pass filter 130 and a continuous-time low-pass filter 140, thereby reconstructing the analog audio signal. However, 1-bit sigma-delta digital-to-analog converters prone to instability and high clock rate issue that cause application limitation on the DAC applications that require high resolution and high bandwidth.
Further, substantial out-of-band quantization noise is generated during the sigma-delta modulator stage such that the digital-to-analog converters output need high-order low-pass filters to filter out such noise to sufficient low to avoid slewing phenomenon generated by the audio amplifier, thereby inducing the inter-modulation and harmonic distortion. As known, those will influence the output quality, seriously. Multi-bit sigma-delta digital-to-analog converters (including MASH DACS) can resolve aforementioned instability, high clock rate issue and reduce the stage required on the low-pass post-filter.
In general, the bit number of the multi-bit sigma-delta DAC is less than 5 bits (including 5 bits) after the modulation by a sigma-delta modulation, and therefore, the SCF stage for extremely high level out-of-band quantization noise power can not be omitted. The low-noise SCF design becomes the key for overall performance of sigma-delta DACS.
Moreover, in multi-channel applications, the SCF of traditional sigma-delta DAC is unlikely to be used mutually, so that how to reduce the cost to increase competitiveness is an issue.
In view of above description, the present invention provides a system and output method of novel multi-channel audio DACs.